Export performance optimization (#475)

* recreate metadata path if needed, before exporting data

vendor/github.com/modern-go/concurrent/go_above_19.go

@@ -0,0 +1,13 @@
+//+build go1.9
+
+package concurrent
+
+import "sync"
+
+type Map struct {
+	sync.Map
+}
+
+func NewMap() *Map {
+	return &Map{}
+}